The present invention relates broadly to high-strength, thin-walled catheters, and more specifically to single-piece, extremely thin-walled, inflatable catheters formed with an oriented polymer having an extremely high tensile strength.
Many diagnostic and/or therapeutic catheters and other medical instruments are often provided near their distal ends with inflatable thin-walled balloons for dilating a particular body part once such device is properly positioned in the body. The balloon is usually separately formed as a sleeve or segment having an enlarged middle section and a reduced diameter at its opposite open ends. The ends are formed so that they can be secured around the periphery of the shaft of the catheter body so as to form a sealed balloon structure. Typically, the balloons collapse about the shaft of the catheter extending through the balloon portion so that the catheter can be directly intubated, or moved through a guide tube previously intubated, in the body. Usually, the balloon portion of the catheter is moved beyond the guide with the balloon portion at the site of interest. Balloon segments are made as thin as possible so that it provides minimal contribution to the overall outer diameter of the catheter when the balloon is deflated. This minimizes the internal as well as the external diameter of the guide tube. Typically, the guide tube must be sufficiently stiff, yet flexible, so that it can be intubated along what sometimes are tortuous paths within the body. See, for example, my U.S. Pat. No. 4,820,349 directed to "Dilatation Catheter With Collapsible Outer Diameter" and issued Apr. 11, 1989. One common application of thin-walled balloon catheters is in the balloon angioplasty procedure wherein the balloon segment at the distal end of the catheter is inflated inside a partially blocked artery section in order to reduce the blockage. Balloon catheters also have applications in other medical procedures involving insertion into blood vessels and other body cavities. Such applications for thin-walled balloon catheters are well-known in the art.
Of critical importance in all such internal applications for thin-walled balloon catheters is that the balloon must be secured to the shaft of the catheter and be of a sufficiently high-strength so as to be resistant to rupture. During use the balloon is filled with a fluid (liquid or gas) usually through the body of the catheter, and a rupture would lead to leakage of a foreign substance into the blood vessel or other body part with potentially harmful results. In addition, it might then be difficult or impossible to withdraw the catheter without trauma to the surrounding tissue. The danger of rupture is particularly great in connection with balloon catheters inflated with a fluid under elevated pressure. In the case of the balloon angioplasty procedure, where the internal diameter of the catheter is relatively small, not only must the pressure be relatively high to inflate the balloon inside a partially blocked artery section but, in addition, the procedure typically involves several inflation-deflation cycles thereby putting further strain on the catheter.
In response to these special demands, materials and procedures have been developed for making high-strength, thin-walled catheter balloon segments for such applications. For example, U.S. Pat. No. Re. 32,983, reissued Jul. 11, 1989, (originally U.S. Pat. No. 4,490,421 to Levy) describes a polymeric balloon having a burst pressure of at least 200 psi. The Levy patent discloses a process for biaxial orientation of polymeric tubing under certain temperature conditions in order to achieve an improved, high-strength dilatation balloon segment. The biaxial orientation of orientable polymers has been found to create very thin films having extremely high tensile strength, far beyond any properties achievable with prior art processes and materials. Levy's preferred polymer is polyethylene terephthalate (PET) having the desirable trait of being relatively non-compliant at body temperature when the material has been highly biaxially oriented.
In accordance with Levy's process, a balloon segment can be produced by extending a length of PET tubing of appropriate dimensions through a mold and out an opening provided at the bottom of the mold. The open end of the tube extending out the bottom of the mold is pinched or clamped off. The portion of the tube within the mold is then heated to a suitable temperature and drawn (stretched) longitudinally to a length 3-6 times its original length by allowing weights or a mechanical stretching device to pull the closed end of the tube. The portion of the drawn tubing inside the mold is then radially expanded at an elevated temperature by introducing a pressurized fluid into the tube. A further description of the Levy dilatation balloons and process appears in the Journal of Clinical Engineering, vol. 11 (July-August 1986), pp. 291-296. One principal advantage of using PET to make the balloon segment is that the material can be stretched very thin, yet because of the biaxial orientation of the polymer it has an extremely high tensile strength and is capable of withstanding high pressures. Recognition of the special high-strength properties of biaxially oriented cylindrical PET objects dates back to at least 1973 when U.S. Pat. No. 3,733,309 was issued to Wyeth et al. directed to high-strength plastic bottles produced using a biaxial orientation process.
More recently I have been issued U.S. Pat. No. 4,820,349 directed to "Dilatation Catheter With Collapsible Outer Diameter," and mentioned above. In the preferred embodiment described in this patent, the balloon segment is made from PET. In a related application, filed in my name and published Jul. 13, 1988 by the European Patent Office as Printed Specification No. 0274411 under the title "Thin Wall High Strength Balloon and Method of Manufacture," I describe the use of thin wall tubings combined with high-stretch ratios and heat setting to provide balloon segments having thin, strong and flexible properties. All of the aforementioned patents and related literature are incorporated herein by reference.
A remaining problem with the balloon segments and associated catheters made in accordance with the prior art, including my previously described inventions, is the adhesive joints at which the balloon segment is attached to the inner shaft. First, it is relatively costly, cumbersome and time-consuming to effect the adhesive seals. Each seal must be complete and perfect in order to avoid leakage of the pressurizing fluid used to inflate the balloon. The adhesive joints represent points of discontinuity and weakness along the outer skin of the catheter. The flexibility of the portion of the catheter provided with the adhesive is reduced due to the relative stiffness of the cured adhesive. The adhesive must meet demanding performance specifications including adhesion to both the high-strength polymeric balloon segment material as well as to the inner shaft extending through the balloon segment. The adhesive must be non-toxic, non-allergenic, and not susceptible to deterioration or chemical attack under conditions of use. Welding or heat sealing pre-formed catheter balloon segments to another catheter element are alternative approaches that do not require adhesives but are also difficult and relatively expensive. Such procedures also invariably lead to partial loss of biaxial orientation of the balloon segment at the fusion points producing weaknesses in the final product. For all of these reasons, it would be highly desirable to provide thin-walled balloon catheters that are free of any adhesive or welded joints.
U.S. Pat. No. 4,254,774 to Boretos specifically recognizes the desirability of forming a balloon catheter such that "the balloon is a continuous, non-interrupted integral part of the catheter wall, free of seams or joints," (col. 1, lines 7-9). Boretos addresses this problem by applying internal fluid pressure to heated plastic tubing in order to create a balloon at one end thereof. Only the balloon portion of the tubing is stretched. The remaining part of the tubing, i.e. the catheter shaft, remains thick and relatively low strength, especially if made with any one of the specific materials identified by Boretos. The balloon is adapted to be invaginated into the shaft of the catheter prior to inserting the catheter into the body passageway of interest. In one embodiment, Boretos uses a mold to better shape the balloon portion of his catheter. The Boretos process, however, involves the manufacture of extremely small catheters having diameters of 5 to 40 mils with relatively thick walls of 2 mils to produce balloons of 100 mil diameters for traversing vessels of 1 mm. (0.03937 inches or 39.37 mils). The balloon portion of the catheter is relatively elastic because the catheter is utilized for partial or total occlusion of a passageway and may be used to deliver substances to a particular site through a performed hole in the inflated balloon. The catheter body or shaft remains relatively thick for its diameter so as to maintain sufficient stiffness so that it can be intubated within a body passageway, and yet sufficiently flexible so that it can follow a tortuous path through the body. While the balloon portion of the Boretos catheter is stretched there is no recognition in Boretos of the special advantages of biaxial orientation of orientable polymers under specific conditions, and no means is suggested by Boretos for carrying out a biaxial orientation of the entire body of the catheter. It is well known that tubing of virtually all unoriented or unreinforced thermoplastic materials has a radial tensile strength of less than 10,000 psi, most being less than 5,000 psi.
U.S. Pat. No. 4,411,055 to Simpson et al. also discloses an embodiment for a dilating catheter assembly "in which an inflatable or distensible balloon-like portion is formed integral with one of the tubular members forming the dilating catheter assembly," (col. 2, 11. 15-18; also, col. 10, lines 32-38). Although the disclosure in Simpson is incomplete with regard to their description of such an embodiment, it is clear that Simpson et al., as in the case of the Boretos patent, do not recognize or discuss the special advantages of using biaxially oriented polymers, nor is any means suggested for carrying out a uniaxial or biaxial orientation of the resulting integral structure.
Finally, guide catheters for receiving and guiding diagnostic and/or therapeutic catheters, must be sufficiently stiff so that it can be inserted into and moved through a particular part of the body, yet sufficiently flexible so that it can follow tortuous paths through the body. With the increased use of laser and ultrasound delivered through a diagnostic and therapeutic catheter, such as suggested in U.S. Pat. No. 4,821,731 (Martinelli), it is desirable to manufacture a guide catheter having a wall sufficiently thin and made of a suitable material transmissive to ultrasound and infrared laser radiation produced, for example, by a YAG laser frequently used for surgical procedures. While PET is a suitable material, it must be at most 1.5 mils thick.